Structural stability of Ga2O3 polymorphs on α-Al2O3(0001) substrates: Insights from surface and interface energies

The structural stability of Ga2O3 thin films grown on α-Al2O3(0001) substrates is systematically investigated using density functional theory calculations, with particular emphasis on interfacial energetics, surface stability, and strain relaxation effects. The calculations for Ga2O3/Al2O3 interfaces demonstrate that the α-Ga2O3/α-Al2O3 interface is energetically more favorable than the corresponding β-phase interface over a wide range of gallium chemical potentials, indicating a strong thermodynamic driving force for α-phase nucleation on sapphire substrates. The influence of lattice mismatch is further examined by considering slip-plane generation. Although strain relaxation reduces elastic energy, the introduction of slip planes increases the interfacial energy, highlighting the competition between strain relief and defect formation. The surface energy calculations reveal that hydroxyl (OH) termination substantially stabilizes α-Ga2O3 surfaces, underscoring the importance of surface chemistry under hydroxyl-rich growth environments, such as mist chemical vapor deposition. By combining the calculated interface and surface energies, the free energy of Ga2O3 thin films is formulated as a function of film thickness. The analysis shows that the α phase is stabilized in the ultrathin regime due to its lower interface energy, whereas strain relaxation and stability favor the β phase as thickness increases. Surface hydroxylation further modifies the critical thickness for phase transition. These findings provide fundamental insight into phase selection and interfacial stability during Ga2O3 heteroepitaxy on α-Al2O3(0001) and offer practical guidance for optimizing thin-film growth conditions.